The present invention relates to the distribution of video signals, and more specifically to a scheme for thwarting piracy of subscription television signals and/or the provision of a data service in an analog television signal by inserting supplemental burst signals into video scan lines.
Techniques for scrambling the video portions of television signals are well known. For example, U.S. Pat. No. 3,813,482 to Blonder discloses a system for transmitting television signals wherein the video is scrambled by suppressing vertical or horizontal synchronization pulses to produce a shifting or rolling scrambled picture. In U.S. Pat. No. 4,542,407 to Cooper, et al., apparatus is disclosed for scrambling and descrambling television programs in which the horizontal synchronization information is suppressed at a cable television headend, and then regenerated by a subscriber's cable television converter. U.S. Patent No. 4,095,258 to Sperber, U.S. Pat. No. 4,163,252 to Mistry, et al., and U.S. Pat. No. 4,571,615 to Robbins, et al. describe apparatus for decoding scrambled television signals.
The economic viability of subscription television programming, including cable television and satellite television services, is dependent on the ability of the transmitter to encode or scramble a television signal so that an unauthorized receiver will not be able to receive a viewable television picture. As indicated in the patents cited above, a common technique that has been employed to scramble video signals is to suppress the horizontal synchronization pulses below the average value of the video level. This causes the television receiver to unsuccessfully attempt to lock horizontally on random video peaks rather than on the horizontal synchronization pulses. The loss of effective horizontal synchronization prevents the receiver from properly utilizing the color burst signal associated with the horizontal synchronization pulse, so that color reproduction is also faulty.
In order for a receiver to be able to view the scrambled video signal, the suppressed synchronization pulses must be restored. Two techniques are commonly employed to allow the receiver to recover the suppressed sync and timing information. In one technique, a timing pulse is amplitude modulated on the FM audio carrier of the television signal, which is then detected in the audio portion of the receiver and used to generate the timing signals necessary to descramble the received video signal. In another known technique, some portion of the sync timing pulses, such as during the vertical interval, is transmitted without suppression, i.e., "in the clear." The receiver phase locks to the clear or unsuppressed sync portion to create the required synchronizing and timing information for descrambling the video portions of the signal.
Neither of these known techniques are immune to unauthorized recovery by "pirate" descramblers. Such descramblers are known that will restore usable synchronization to virtually all known prior art sync suppressed video signals. Such devices typically use the color burst component of the video signal as a "key" to recover all other necessary sync and timing information.
All NTSC (National Television Systems Committee) video signals contain a color burst component at 3.58 MHz. The horizontal frequency of the video signal is related to the burst by a factor of 227.5. Therefore, by extracting the 3.58 MHz color burst components from a received scrambled video signal, a horizontal synchronization pulse can be generated and inserted into the video signal where the original pulse has been removed or suppressed. A television receiver can then lock to the new sync pulse so that video information can be viewed.
It would be advantageous to provide a scheme that would preclude the use of the color burst components to recover timing information for descrambling a received video signal. Such a system should not interfere with the descrambling of scrambled signals by the millions of authorized descramblers already installed in the field. One use of the present invention is to provide such a scheme.
It would also be advantageous to provide a scheme for transmitting data in an analog television signal. The digital revolution has provided and will continue to provide many uses for data carried along with a television signal. For example, new data and information services such as teletext, advertising, program information and the like can be provided once a source of data is available in a television signal. Data for the operation of various television receiver functions may also be necessary, and a means for transporting such data to the receiver is required.
Although it is known how to carry data in a single scan line of a television signal, such as by replacing an active video waveform with a waveform representative of digital data, it has not heretofore been possible to easily carry data in an analog television signal outside of the "information portions" of the video scan lines. It is an object of the invention to provide a means for transporting binary information within an analog television signal without significantly degrading the video output that is to be derived from the signal.
The present invention provides apparatus for inserting supplemental burst signals into various lines of an analog television signal in order to prevent the regeneration of synchronization and timing information from the standard color burst signal and to provide a means for carrying data within the analog television waveform. These and other advantages will be apparent from the following description and claims.